Concrete including sections that present limited access to

Concrete occupies
unique position
among
the
modern
construction
materials, Concrete
is a material used in building
construction, consisting of
a hard, chemically inert
particulate substance, known as
a aggregate
(usually
made for different types of
sand and gravel), that
is bond by cement
and
water.

Self – compacting
concrete (SCC) is
a high – performance
concrete that
can flow under
its
own weight
to completely fill
the
form work and self
consolidates without
any mechanical vibration.
Such concrete an accelerate the placement, reduce the labor requirements needed for consolidation,
finishing
and
eliminate
environmental pollution.
The
so called first generation SCC is used
mainly for repair application
and
for
casting
concrete in
restricted areas, including
sections that present
limited access to vibrate.
Such
value added construction material
has
been used in applications justifying the higher material
and
quality
control
cost when considering the simplified placement
and
handling requirements of the
concrete.

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The successful production of
self – compacting
concrete (SCC) for use, is
depended
on arriving at an appropriate balance
between the yield
stress and the viscosity
of the paste. Specially formulated high range water
reducers
are
used
to reduce the yield stress
to point to allow
the designed
free flowing characteristics
of the concrete. However, this alone may result
in segregation
if the viscosity
of the paste
is not sufficient to support the aggregate
particles in suspension.

The process
of selecting
suitable
ingredients
of concrete
and
determining
their
relative
amounts with an objective of
producing
a concrete of
required
strength, durability,
and
workability
as economically as possible
is termed
as concrete mix
design.

 

The Mix Design for concrete M30 grade is being done as per the Indian Standard Code IS: 10262-1982.CHAPTER – 1INTRODUCTION  1.1  GENERALSelf – compacting
concrete (SCC) is
a fluid
mixture,
which
is suitable for placing difficult conditions and also in
congested reinforcement, without vibration.
In principle, a self
– compacting or
self – consolidating concrete
must: v Have a fluidity that allows self – compaction without external energyv Remain homogeneous in a form during and after the placing process and v Flow easily through reinforcement Self – consolidating concrete has recently been used in the pre – cast industry and in somecommercial applications,
however the relatively
high
material cost
still hinders
the
wide spread use of such
specialty concrete
in various
segments of
the
construction
industry,
including commercial and residential construction. Compared with
conventional concrete of similar mechanical properties, the material cost of
SCC is more due to the relatively high demand of cementation materials and
chemical admixtures including  high – range water reducing
admixtures (HRWRA) and viscosity enhancing admixtures (VEA). Typically, the
content in Cementation materials can vary between 450 and 525 Kg/m3 for SCC
targeted for the filling of highly restricted areas and for repair
applications. Such applications require low aggregate volume to facilitate flow
among restricted spacing without blockage and ensure the filling of the
formwork without consolidation. The incorporation of high volumes of finely
ground powder materials is necessary to enhance cohesiveness and increase the
paste volume required for successful casting of SCC. Proper selction of finely
ground materials can enhance the packing density of solid particles and enable
the reduction of water or HRWRA demand required to achieve high deformability.
It can also reduce viscocity for a given consistency especially in the case of
SCC made with relatively low Water – Binder ratio. Reducing the free water can decrease the
VEA dosage necessary forstability.
High binder content typically includes substitutions of cement
with 20
to 40% fly ash or GGBS and,
in some
cases low contents
of micro
silica employed.
The
cost of
SCC can
be reduced through the selection of adequate concrete – making materials and admixture constituents,
including partial
substitutions of cement
and
supplementary Cementations
materialsby readily available fillers.Regardless
of its binder composition, SCC
is characterized by its low yield
value to
secure high deformability,
and
moderate
viscosity to provide
uniform
suspension
of solid
particles,
both during casting
and
thereafter until
setting.
The
mixture proportioning
of SCC
to simultaneously meet the various performance
requirements at
minimum cost
involves the optimization of several mixture
constituents that
have
a marked
influence
on performance.
This
process
is quite complex and can be simplified
by understanding
the
relative
significance of
various mixture
parameters
on key properties
of SCC. This includes deformability,
passing
ability, filling capacity
and segregation resistance.As with
any new technology, there was
clearly
a learning
curve to overcome,
and
refinement
of the materials
and
mix proportions used took care to finally achieve
optimum performance.
In Japan, self – compacting concretes are divided
into
three
different types
according to the composition of the mortar:v  Powder
typev  Viscosity
– modifying agent (stabilizer) typev  Combination
typeFor the powder
type, a high proportion
of fines
produce
the
necessary
mortar volume, while
in the stabilizer
type, fines
content
can be in the
range admissible
for
vibrated concrete. The viscosity required
to inhibit segregation will then
be adjusted
by using
a stabilizer (Kosmatka
et al., 2002). The combination type
is created
by adding a small
amount
of stabilizer to the powder type to balance the moisture fluctuations
in the manufacturing
process.The SCC essentially
eliminates the need for vibration to
consolidate
the
concrete.
This
results
in an increase in
productivity,
a reduction
in noise exposure
and
a finished product
with
few
if any external blemishes such as
“bug holes”.
However, after completion
of proper
proportioning,
mixing, placing, curing and consolidation, hardened concrete becomes a strong, durable, and practically impermeable building material that requires no maintenance. 1.2 BENEFITS AND ADVANTAGESAt present self – compacting concrete (SCC) can be classified as an advanced construction material. The SCC as the name suggests, does not require to be vibrated to achieve full compaction. This offers benefits and advantages over conventional concrete. Ø  Improved quality of concrete and reduction of onsite repairs.Ø  Faster construction times.Ø  Lower overall costs.Ø  Facilitation of introduction of automation into concrete construction.Ø  Improvement of health and safety is also achieved through elimination of handling of vibrators.Ø  Substantial reduction of environmental noise loading on and around a site.Ø  Possibilities for utilization of “dusts”, which are currently waste products and which are costly to dispose of.Ø  Better surface finishes.Ø  Easier placing.Ø  Thinner concrete sections.Ø  Greater Freedom in Design.Ø  Improved durability, and reliability of concrete structures.Ø  Ease of placement results in cost savings through reduced equipment and labor requirement.Ø  SCC makes the level of durability and reliability of the structure independent from the existing on – site conditions relate to the quality of
labor, casting and compacting systems available.Ø  The high resistance to external segregation and the mixture self – compacting ability allow the elimination of macro – defects, air bubbles, and honey combs responsible for penalizing mechanical performance and structure durability. 1.3 DEVELOPMENTS OF SELF – COMPACTING CONCRETEFor several
years beginning in
1983, the problem of
the
durability
of concrete
structures was
a major topic of interest in
Japan.
The
creation
of durable concrete
structures requires
adequate compaction
by skilled workers.
The
designs
of modern reinforced
concrete structures become more advanced,
the
designed
shapes
of structures
are
becoming increasingly complicated
and heavy reinforcing is
no longer unusual.
Furthermore, the gradual
reduction
in the number of skilled workers in
Japan’s construction industry
has led to a similar
reduction
in the quality of construction work. One
solution
for
the
achievement
of durable
concrete structure
independent of the quality of
construction
work
is the employment of
self – compacting concrete, which can be compacted into every corner of
a form work,
purely
by means of its own
weight
and
without the need for
vibrating compaction. Okamura
proposed
the
necessity of
this
type
of concrete
in 1986. Studies
to develop
self – compacting
concrete,
including a fundamental
study on
the workability
of concrete, have
been
carried
out
by “Ozawa and Maekawa”
at the university of Tokyo.The prototype of
SCC was first completed in
1988 using materials
already
on the market.
The proto type performed
satisfactorily with
regard
to drying and
hardening shrinkage,
heat
of hydration, denseness after hardening, and other properties. This concrete
was
named
“High Performance Concrete” and was defined as
follows at
the
three
stages of concrete: 1.      Fresh    : Self – Compactable.2.      Early age : Avoidance of initial defects3.      After hardening: Protection against external factors. “High Performance Concrete” was defined as a concrete with high durability due to a low water–     cement ratio by professor Aitcin et al (Gangneetal 1989). Since then, the term high performance concrete has been used around world to refer to high durability concrete. Therefore, H.Okamura and M.Ouchi, the authors, of an invited paper on SCC for JACT 2003 have changed the term for the proposed concrete, for their work, to “Self – compacting High performance Concrete”. 1.4 MECHANICAL CHARACTERSTICSØ  Characteristic compressive strength at 28 days shall be 25 – 60 Mpa.Ø  Early age compressive strength shall be 5 – 20 Mpa at 12 – 15 hours (Equivalent age at200 C)Ø  “Normal” creep and shrinkage  1.5 HOW DOES IT WORK? A self consolidating must: Ø  Have a fluidity that allows self – consolidation without external energy.Ø  Remain homogenous in a form during and after the placing process andØ  Flow easily through reinforcementTo achieve these performances, Okamura redesigned the concrete mix design process. His mix design procedure focused on three different aspects:Ø  Reduction of the aggregate content in order to reduce the friction, or the frequency of collisions between them increasing the overall concrete fluidityØ  Increasing the paste content to further increase fluidityØ  Managing the paste viscosity to reduce the risk of aggregate blocking when the concrete flows through obstacles.In rheological
terms, even
though a significant
amount
of research tends
to show
that SCC’s viscosity varies
with the
shear rate
and
acts
as a pseudo plastic
material,
SCC is
often
described as Bingham fluid (visco elastic)
where
the
stress/shear rate
ratio
is linear and characterized by two constants – viscosity and
yield stress.Back to the performance based definition of SCC, the self
– consolidation is
mainly
governed by yield stress,
while the viscosity will
affect the homogeneity
and
the
ability to flow
through reinforcement. As
the
SCC viscosity can be
adjusted depending
on the
application,
the
yield stress remains significantly lower
than other
types of
concrete in
order
to achieve self – consolidation.1.6 APPLICATIONS Applications of Self Compacting Concrete in Japan. Current conditions on application of self – compacting concrete in Japan.After
the
development
of the
prototype of self – compacting concrete
at the University of Tokyo, intensive research
was begun
in many places,
especially
in the research institutes
of large construction
companies. As a result, self
– compacting
concrete has been
used
in many practical structures. The first application of
self – compacting concrete
was in
a building in
June
1990. Self
– compacting concrete
was then
used
in the towers
of a pre stressed
concrete cable – stayed bridge in 1992. Since
then,
the
use of self
compacting concrete
in actual structures
has
gradually increased.
Currently, the main
reasons
for
the
employment
of self
– compacting concrete
can be summarized as follows. 1.      To shorten construction period2.      To assure compaction in the structure: especially in confined zones where vibrating compaction is difficult.3.      To eliminate noise due to vibration: especially at concrete products plants.The production of self-compacting concrete as a percentage of Japanese ready mixed concrete, which accounts for 70% of total concrete production in Japan, is only 0.1%. The current status of self-compacting is ‘special concrete ‘rather than ‘standard concrete’. Other applications of self-compacting concrete are summarized below. Ø  Bridge (anchorage, arch, beam, girder, tower, pier, joint between beam and girder)Ø  Box culvertØ  buildingØ  concrete filled steel columnØ  tunnel(lining, immersed tunnel, fill of survey tunnel)Ø  dam(concrete around structure)Ø  concrete products (blocks, culvert, wall, water tank, slab and segment)Ø  diaphragm wallØ  tank(side wall, joint between side wall and slab)Ø  Fire proof. 1.6.1. Large scale constructionSelf-compacting
concrete is
currently
being employed in
various practical structures
in order to shorten the construction period of large-scale
constructions.The anchorages
of Akashi-Kalikyo (Akashi straits) Bridge
opened
in April 1998, a suspension bridge with the
longest span in
the
world (1,991mts),
is a typical example (Kashima 1999).
Self- compacting concrete
was used in the construction
of the two
anchorages
of the bridge. A new construction
system that
makes full
use of
the
performance of
self-compacting concrete
was introduced for the purpose. The concrete
was mixed
at the batcher plant next
to the
site,
and
was then pumped out
of the plant.
It was
transported 200mts
through
pipe
to the casting site, where the pipes were arranged
in rows 3 to 5mts
apart.
The
concrete was
cast
from gate
valves
located at 5mts intervals along the pipes. These valves were automatically
controlled
so that
the
surface level of the
cast
concrete could
be maintained.
The
maximum
size of
the
coarse aggregate
in the self-compacting concrete
used
at this site
was 40mm the
concrete fell
as much
as 3mts, but segregation
did
not
occur, despite
the
size of
coarse aggregate. In
the
final analysis the use
of self-compacting concrete
shortened
the
anchorage construction period by 20% from
2.5 to2 years.Self-compacting
concrete was
for
the
wall
of a large LNG
tank belonging
to the Osaka gas company. The
adoption of
self-compacting concrete
in this particular project had the following merits.1.      The number of lots decreased from 14 to 10 as the height of one lot of concrete was increased.2.      The number of concrete workers was reduced from 150 to 50.3.      The construction period of the structure decreased from 22 months to 18 months.In addition, a rational acceptance test for self-compact ability at the job site was newly introduced. The concrete casting was complete
in
June 1998. 1.6.2. Concrete productsSelf – compacting
concrete is
often
employed in
concrete products to eliminate
vibration
noise. This improves
the
working
environment at
plants
and
makes the location of
concrete products plants in urban
areas possible.
In addition,
the
use of self
– compacting concrete
extends
the lifetime
of
mould for
concrete has
been gradually increasing.1.7. NECESSITY FOR NEW STRUCTURAL DESIGN AND CONSTRUCTION SYSTEMSSelf – compacting
concrete saves
the
cost of vibrating
compaction and
ensures the
compaction of the concrete in
the
structure.
However,
total
construction
cost cannot
always
be reduced,
exceptin large – scale
constructions. This is
because
conventional
construction systems are essentially designed
based
on the
assumption that vibrating
compaction of concrete is
necessary.Self – compacting
concrete can
greatly
improve construction systems previously
based
on conventional concrete that
required vibrating
compaction.
This
sort of
compaction,
which
caneasily cause segregation,
has
been an
obstacle to the rationalization of
construction
work. Once this obstacle is
eliminated, concrete
construction
can be rationalized
and
a new construction system including form work,
reinforcement, support and structural design, can be developed.

One example of this is
the
so called sandwich structure, where
concrete is
filled into a steel shell. Such a structure
has
already been
completed
in Kobe, and could
not
have been achieved
without the development
of
self – compacting
concrete (Shishido
et al, 1999).